Implantable leads have numerous applications and are commonly used in medical devices to record electrical activity and/or stimulate a target site. An example of a widespread use of implantable leads in medical devices is in pacemaker or implantable cardioverter-defibrillator (ICD) implantations. Devices such as pacemakers are implanted into the heart as part of artificial cardiac pacing or cardiac resynchronization therapy, which involves generating electrical impulses that are carried by the pacing lead to the heart tissue fibers, signaling them to contract and relax properly. The implantable lead has a distal end and a proximal end. The distal end makes contact with the heart tissue and the proximal end is configured to make contact with the pacing pulse generator. Furthermore, at each end there are two electrical terminals. In particular, at the distal end there is a negative and a positive terminal, and at the proximal end there is a corresponding negative and a positive terminal. Therefore, the negative terminal of the distal end is electrically coupled to the negative terminal of the proximal end, and the positive terminal of the distal end is electrically coupled to the positive terminal of the proximal end. The positive terminals and negative terminals are electrically isolated from each other.
Pacemaker and implantable cardiac device (ICD) leads are anchored to tissue using a fixation mechanism. The lead is typically introduced into the venous system under the patient's collarbone, and its distal end is advanced toward the patient's heart by guiding the lead until the distal end has reached the desired heart wall location in the atrial or ventricular chamber.
There are two common types of fixation mechanisms used to anchor an ICD lead to tissue: active and passive fixation. The passive fixation mechanisms have a plurality of flexible tines that protrude from the distal end of the implantable lead. When the distal end of the lead is pushed into the cardiac tissue the tines latch onto the tissue in order to secure the fixation tip in place. Active fixation mechanisms, by contrast, have a corkscrew-like apparatus at the distal end of the lead which is retracted while the lead is guided to the heart. Once the clinician has determined the desired fixation position (using various imaging technologies and tactile feedback), current practice of anchoring the distal end of the lead to the heart wall involves using a disposable tool to turn a fixation pin at the proximal end of the lead. The clinician turns the pin a predetermined number of times causing advancement of the corkscrew-like apparatus at the distal end into the tissue. Using this method, the clinician attempts to ensure that the lead has an adequate hold on the tissue. The success of anchoring is determined based on several factors including tactile feedback, electrical measurements, and the overall experience of the clinician.
The tactile feedback method typically involves gently tugging on the proximal end of the lead, prior to taking electrical measurements. If the distal end remains secured, the clinician may be satisfied with anchoring. The tactile feedback method relies on subjective standards. The electrical measurement includes connecting the negative and positive terminals of the proximal end to an electrical measurement device. Here, various signals are communicated through the proximal end terminals to the distal end terminals enabling the measurement of conductivity, impedance, electrocardiogram (ECG) amplitudes, pacing thresholds, maximum output and slew rate at the distal end. While the electrical measurement method provides some objective data, it may be difficult to correlate the electrical data obtained from the measurement to how well the lead is anchored in the tissue. The clinician preference may include viewing the lead anchoring in the tissue via various imaging technologies, the tactile feedback method, and/or the electrical measurement method.
The above-described pacemaker implantation methods, however, can result in complications. Common complications include lead malposition (i.e., situations where the lead is not properly placed, potentially resulting in undesired lead penetration into the tissue) or migration (i.e., the lead has moved from the desired location). Lead migration can result in undesirable complications indicated by changes in conductivity which can increase pacing thresholds required to stimulate the heart, decreased sensing ability of the pacemaker, and can thus lead to decreased device performance or even life-threatening consequences. Perforation of the heart wall caused by lead penetration to the tissue can result in various complications, including pericardial effusion.
Aside from pacemakers and ICDs, there are a number of other applications of implantable leads. These applications include: spinal cord stimulators; spinal fusion stimulators; bone growth stimulators; implantable electrocardiogram systems; neuromodulation systems (for example, to be used in cochlear implants, vagus nerve stimulators, deep-brain stimulators, sacral nerve stimulation, implantable electromyography recording devices, migraine treatment, spinal cord injuries, and pain management); and subarachnoid stimulators. All these applications may include a lead anchoring in a respective tissue as described above. Similarly, these other applications may suffer from the same or similar complications as described above.
In view of the foregoing, there is an unmet need for a reliable and efficient system which can provide effective feedback for ensuring proper anchoring of an implantable lead into tissue.